Rolling plants disposed in a line with a continuous casting machine which produces thin slabs, or “thin slab casters”, are known.
Such plants can be planned and configured for a substantially continuous rolling process, or “endless”, in which the cast product is rolled in a rolling train which is placed immediately at the exit of the continuous casting machine with which it is in direct contact.
The fact that the rolling train is directly attached at the exit of the continuous casting machine in the endless process allows temperature not to be lost and, moreover, to exploit to the full the heat in the cast product and the low resistance to pressure in the first two-three rolling stands inasmuch as re-crystallization has not yet completely taken place, with consequent saving of energy in the rolling step.
The rolling process of the endless type ensures the possibility of producing ultra-thin strip (for example from 0.7 to 0.9 mm) in that sequences are begun by producing thicknesses from 1.5-3.0 mm then progressively decrease to 0.7-0.9 mm.
Unfortunately the endless process, like the one shown for example in the patent EP 1868748, the layout plan of which is shown in FIG. 1, is very rigid for the reasons given below.
The production of some qualities of steel (for example, peritectic steel, high carbon content steel, silicon steel, API steel) obliges, for metallurgic and qualitative requirements, to lower the maximum speed of continuous casting and, consequently, the mass-flow falls below the minimum value needed to obtain the temperature of at least 850° C. in the last stand of the finishing train, thus rendering endless rolling impracticable for a vast range of thicknesses from 0.7 to 4.0 mm, despite an induction heating located on the train.
Moreover, as the rolling train is located immediately at the exit of the continuous casting machine in the endless process, there is no possibility of having an intermediate buffer between the two rolling and casting processes which are rigidly connected. Therefore each minimum stoppage of the rolling mill and/or the strip winding machines, for example due to a programmed change of the rolling rolls, in order to carry out controls, due to accidents, sudden interruptions or minor breakdowns, requires that the continuous casting process and also that of the steel works upstream is stopped, with a loss in production.
This characteristic of the endless process that it does not have any buffer has the following consequences:
the use factor of the casting-rolling plant, but also that of the steel works upstream, is reduced by 5÷6%;
the yield of the plant (that is the ratio between weight of finished product and weight of liquid steel in the tundish to produce a ton), decreases by 1.2÷1.3% because of the loss of material which is a result of the scrapping of the steel present in the tundish at the exit of the continuous casting machine.
Moreover, the endless process does not allow to insert a second casting line so as to increase the productivity of the plant.
Finally, the endless process has very little flexibility in the production changes (slab width and thickness).
On the contrary, the layout solutions using the thin slab caster of the semi-continuous type provide that the casting machine and the rolling mill are connected in line by a tunnel furnace for heating and/or maintenance which also acts as an accumulation store for the slabs when it is necessary to overcome an interruption of the casting process, because of incidents or because of a programmed roll change, in this way avoiding losses of material and of energy and above all, avoiding an interruption of the casting.
In the case where, in a semi-continuous process where the length of the slab corresponds exactly to the material needed to form a coil of the desired weight, the process is called “coil-to-coil”.
In the case where the length corresponds to a multiple of the length needed to form a coil of the desired weight, the so-called super-slab, then the process is called “semi-endless”.
We shall now give a summary to clarify the characteristics of the three processes so far considered.
Endless: the process happens in a continuous manner between casting and rolling mill. The cast slab feeds the rolling train directly and continuously. The coils are produced in continuous rolling. The individual coils are formed by means of a cut from the quick shears before the winding reels. There are no entrances in the rolling train.
Semi-endless: the process happens in a discontinuous manner between casting and rolling mill. The super-slab, equivalent to “n” (from 2 to 5) normal slabs, is formed at exit from casting by the cut of the pendulum shears. “N” rolling coils are produced from the relative super-slab at a time. The individual coils are formed by a cut from the quick shears before the winding reels. For every sequence of “n” coils produced there is an entrance in the rolling train.
Coil-to-coil: the process happens in a discontinuous manner between casting and rolling mill. The individual slab is formed at exit from casting by the cut of the pendulum shears. One coil at a time is produced in rolling from the relative starting slab. For every coil produced there is an entrance in the rolling train.
Currently technology provides various solutions, mainly in the bibliography and literature of patents, which have provided various types of plants and processes for rolling flat products, each of which is characterized by one of the modes cited above, that is, “endless”, “semi-endless” or “coil-to-coil”, which in general are actuated individually or at most only two per plant.
The existing solutions have pros and cons but do not however manage to satisfy to a great extent the needs of a plant which is both flexible and versatile so as to serve the market competitively.
In particular the processes currently existing have the following characteristics which are also summarized in the comparison table shown in FIG. 5:                Endless: optimal for producing ultra-thin thicknesses from 0.7 to 0.9 mm in that it eliminates the entry of the head of the bar in the stands, therefore with lower wear on the rolls and with fewer risks of blockages, it allows a stationary rolling, but on the other hand it cannot produce some types of steel, it has a low use factor of the plant, a low yield and it does not have the possibility of inserting a second line to increase production;        Coil-to-coil: allows to produce the whole range of castable steels with a thin slab caster, it has high use factor of the plant and high yield. On the other hand, it cannot produce thicknesses below 1.0 mm because of the difficulty the strip has in entering the last rolling stands because it is thin and therefore inconsistent.        Semi-endless: is optimal for producing thin thicknesses up to 0.9 mm, it allows to produce the whole range of castable steels with a thin slab caster, it has a high use factor of the plant and high yield. On the other hand, it has a low productivity in the production of ultra-thin strip (0.7-0.9 mm) in that the process necessarily entails that the first and the last coil of the slab are produced with increased thickness; it reduces (by ¼ or ⅕), but does not eliminate the problem of the entrance of the bar into the stands of the rolling train, and finally, it increases the problems of entrance of the strip to the winding reels in that the speeds of advance of the strip are very high compared to the endless mode.        
The development of casting technology, particularly by the Applicant, with the introduction, for example, of high-performance crystallizers and sophisticated techniques of dynamic soft reduction, which allow to increase the casting speed and to keep it substantially constant on a wide range of thicknesses, for example from 30 to 140 mm, is beginning to allow to hypothesize new plant and process solutions which considerably increase the flexibility of the plant and to obtain a very high productivity together with a high final quality and to obtain extremely reduced thicknesses.
It is known that the starting cast thickness, given the same casting speed, determines the productivity of the plant, the overall number of rolling stands to be used and, in the case of the “endless” rolling process, the temperature profile from the exit of the continuous casting to the exit of the last finishing stand.
Starting from determinate initial parameters, relating for example to the starting thickness of the cast product, to the final thickness of the rolled product, to the productivity required, the purpose of the present invention is therefore to produce rolling profiles and relative lay-outs of plants capable of producing all the qualities of castable steel with the thin slab technology, together with the available sequences of liquid steel upstream, being able to manage the stopping times of the rolling plant for minor maintenance, roll changes and/or incidents, without ever interrupting the casting process.
The Applicant has devised, developed and tested the present invention to obtain these and other purposes and advantages which will be identified in more detail in the following description.